Process for the manufacture of neutral sodium phosphate



in C. F. BOOTH ET AL FROCESS FOR `THE MANUFACTURE OF NEUTRAL SODUM PHOSPHATE Filed July 19, 1924 @man O--F/lm'l'urza oF kwam. Lcsmp Qucmme SLP. I6E=TRI5C DIU/W PH )5 PHATE um DSE D 5( DIUM P1405 PHATE [QA-no 0.445 View: 76% CRYSTAL 5 @aan M/ REA N/ @am 3 if C f FIC C EFD/VTV.

SPEC? Patented Feb. 5, 1929.

`UNITED `STATES PATENT OFFICE.

CHARLES F. BOOTH AND ARTHUR IB. GERBER, OF ANNISTON, ALABAMA, ASSIGNORS T FEDERAL PIIOSPHORUS COMPANY, OF BIRMINGHAM, ALABAMA, A CORPORA- TION OF ALABAMA.

PROCESS FOR THE MANUFACTURE Application filed July 19,

, handling it.

, is commercially called tri-sodium does not conform to the formula .than the theoretical 410.5

According to the present practice, trilsodium phosphate is generally manufactured by the neutralization of sodium carbonate, which is commerciallycalled soda ash, with phosphoric acid, whereby di-sodium phosphate is produced. After the production of di-sodium phosphate it is causticized by the addition of caustic soda, thus converting the cli-sodium phosphate intotri-sodium phosphate. The resultanttrisodium phosphate .solution is then lcooled in a suitable manner and solid tri-sodium phosphate separated out asa crystalline material. Based on the following chemical equations:

the theoretical -quantities of materials necessary for the production of tri-sodium phosphate are as follows: y

25.7#1-13104 per 100#- tri-sodium phos- 27.8# N 212003 phate.

10.5#INaOH per 100# tri-sodium phoshate.v f f p Actual practice fromsevcral years operation shows that in so far as-the caustic is concerned this quantity is insufficient, and, therefore, it appearsthat the material which phosphate per 100# tri-sodium phosbut contains iii addition NaOH, and the caustic soda Anecessary forl crystallization of tri-sodium phosphate approaches nearer 15# NaOH as given above. ExtensiveJ experiments were con- OF NEUTRAL SODIUM PHOSPHATE.

1924. Serial No. 726,917.

ducted to determine if it were possible to reducethe quantity of caustic necessary in tri-sodium phosphate manufacture as that element is both expensive and objectionable. During this investigation it was discovered that it was possible to produce tri-sodium phosphateby the use of less caustic soda, and the resulting compound was neutral when titi-ated with a standard acid, using methyl orange as an indicator. The titration ratio varies from approximately 0.432 to 0.451, depending upon the concentration of the solution, a high ratio being used for higher concentrations. The necessary caustic is added to conform to the requirement as given in the chemical equation as follows:

This caustic soda requirement or ratio may be judged by a. method of control in which the solution is titrated with a half normal hydrochloric acid solution using phenolphthalein and methyl orange as indicators. When the endpoint for the phenolphthalein is reached, a reading is made of the acid used, and the titration is continued to a slight acid end point with the methyl orange. The reading of the titration of the phenolphthalein is then divided by the sum of the readings and a ratio of 0.432 to 0.451 is the desired. resultant for the optimum conditions to produce the best yield of trisodium phosphate crystals when the solution is cooled. The ratio so determined is one means of deriving a constant as a measure of the caustic present in the solution or crystals. lVhere the term ratio is used herein such method of determination is referred to. In the prepa-ration of neutral tri-sodium phosphate it was soon found that the presence of sodium carbonate had a very great influence on the formation of neutral tri-Sodium phosphate crystals. This influence was to keep trisodium phosphate in solution, as well as to reduce the P205 concentration in any given solution. In addition to keeping the tri-sodium phosphate in solution, the sodium carbonate caused the crystals which formed to be very small, soft crystals. The yield was very low,

and di-sodium phosphate and sodium carbonate with varying ratios, and varying specific gravities, and the 30% yield referred to above was obtained from a solution having a ratio of 0.409 in the solution and a specific gravity of 1.40 at 57 o C. The liquor after the crystals were removed had a ratio of 0.375, and a specific gravity of 1.34; While a solution of the crystals in vater had a ratio of 0.496. This value for the crystals is very nearly what would be expected of pure neutral trisodium phosphate, viz, .0.50. l

If the sodium carbonate be absent, it is possible to crystallize neutral tri-sodium phosphate from a solution at 1.36 speciic gravity at 90 C., and a ratio of 0.445, and get approximately yield. As the specific gravity increases the ratio must also be increased to form neutral tri-sodium phosphate. 1f the specific gravity is too high with respect to the ratio some di-sodium phosphate Will be formed; While if the ratio is too high with respect to the specific gravity, there will be some alkaline trisodium found.

The saturation curve E-F shown in the drawing will illustrate the necessity of Varying the ratio With the specific gravity, and the crystallization area curves A-B and C-D show Within what area neutral trisodium phosphate can be produced. .If-the solutions be kept Within the specified area N, there is no difiiculty in producing tri-sodium phosphate corresponding to the formula Na,PO,.12H2O.

The production of this new compound by our process, ofl'ers distinct economic advan tages over the manufacture of tri-sodium phosphate as produced at the present time, and in addition to this economic feature, a tri-sodium phosphate containing no free al kali is produced Which has distinct advantages for the consumer. The process and equipment of manufacture is similar to that carried on and used in the manufacture of alkaline tri-sodium phosphate.

A similar method of control is employed in alkaline tri-sodium phosphate operation, and the figures obtained usually vary between 0.53 and 0.55. These latter figures Will approximate the result arrived at by the same method of testing the crystals formed from solutions having a titration ratio varying between 0.53 and 0.55, and `the difference between these titrations represents the excess.

of caustic in an alkaline tri-sodium phosphate over that produced by the herein described methods of manufacturing trlsodium phosphate and indicates the excess of caustic present in the alkaline tri-sodium phosphate. After the tri-sodium phosphate solution is tested as described above, it is cooled in any suitable manner and the trisodium phosphate crystals separate out. They may be dried in a centrifuge or any dium carbonate, it is necessary to concentrate the di-sodium phosphate or the tri-sodium phosphate solutionbefore substantial recoveries may be made in the crystallization step. It is evident that if a sufficiently concentrated acid is used it is unnecessary to employ this concentration step inasmuch as the water will be absent from the operation, and'therefore the percentage of tri-sodium phosphate in the solution Will be sufficiently high to permit removal of a substantial quantity of the crystals when the solution is cooled. Therefore, while dilute acid may be used in the manufacture of tri-sodium phosphate, it isV preferable that a concentrated acid of approximately 60% HalEO4 be used to avoid the necessary evaporation step. If the di-sodium phosphate is produced by roasting calcium phosphate with sodium acid sulphate, the resultant (li-sodium phosphate may be treated in a manner similar to that described already.

The following vproportions are given as illustrative of the proportions in making the above described tri-sodium phosphate by our process.

28# commercial soda ash (58% NazO).

11:#- caustic soda (7 6% Naz).

i The acid is neutralized by the soda ash and any compounds such as iron, alumina, and calcium phosphates which are precipitated, are removed from the solution, preferably by filtration. The di-sodium phosphate solution is boiled prior to filtration in order that the carbon dioxide (CO2) may be eliminated. After filtration caustic soda is added and the solution thoroughly mixed. The mixture is then tested according to the previously outlined method and if suiicient caustic has not been added, an addition is made until the titration gives the desired figure; While if an excess of caustic has been added, di-sodium phosphate solution is added to reduce the figure to the desired point. The same equipment can be used for the subsequent cooling, crystallization, de-watering and drying of this product as for the alkaline tri-sodium phosphate.

Having thus described our invention, what We claim as nev1 and desire to secure by Letters Patent, is

l. The herein described process for the production of tri-sodium phosphate containing no free alkali which consists in providing a disodium phosphate solution substantially free of sodium carbonate, and adding caustic soda to the solution until the resultant solution has a relation of 'the phenolphtlialein titration to the methyl orange titration of ..432 to .451. 'l y 5 2. The process set forth in claim 1 in which the specific gravity of the solution is maintained above 1.30. 'i

3. In a process of producing tri-sodium hosphate,the steps which comprise titrat- 10 ing the tri-sodium phosphate in solution using phenolphthalein and methyl orange as indicators and adding ycaustic soda to the tri-sodium phosphate solution until the relation of the phenol to the methyl orange titration is from .432 to `.451. 4

In testimony whereof we ax our signatures.

CHARLES F. BOOTH. ARTHUR B. GERBER. 

